Add support for relative paths in Link in XWiki Syntax 2.1

Hi devs,

Someone on the list asked how create a link and an image pointing to a relative reference (that he got using one of XWiki's API since they all return relative URL). We currently don't have a good solution for this and I'd like to propose a solution.

For links
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Make the usage of a prefix that specifies what comes next a generic feature. We have the following needs: 
* link to a URL 
* link to a relative path 
* link to a document 
* link to a mailto 
* link to an attachment

I propose the following syntax:

[[label>>prefix:value]]

where prefix = one of http:, path:, doc:, mailto:, attach:

In addition I propose that when there's no prefix or if the prefix is an unknown prefix then to consider that the reference is a reference to a document (equivalent to doc:).

Pros: 
* Generic
* Ability to add more prefixes later on (for ex ftp:)
* Ability to have virtual wikis which have "reserved" names such as "doc", "http", "ftp", "mailto", etc since you can write [[doc:doc:Space.Page]]
* Ability to write shorthand for documents in the text, such as: "This is a link doc:Space.Page"
* Ability to reference relative paths and write stuff such as {{velocity}}[[label>>path:$xwiki.getURL(something)]]{{/velocity}}

For images
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* Similar to links add the ability to add prefixes in a generic way. We have the following needs: 
* image where the source is at a URL 
* image where the source is at a relative path 
* image where the source is in a document

I propose the following syntax:

image:prefix:value

where prefix = one of http:, path:, attach:

Note that we already support image:http://... so the proposal here is simply to make it generic.

In addition I propose that when there's no prefix or if the prefix is unknown then to consider that the image is to be found in a document (equivalent to attach:).

Same pros as above.